1. Field of the Invention
This invention relates to a distance measuring apparatus of a camera and more particularly to a distance measuring apparatus used for a camera or the like and an active type distance measuring apparatus wherein a spot-like beam as of infrared rays is projected to an object to be photographed and the reflected light from the object is received and used to automatically measure the distance to the object.
2. Related Background Art
As well known, the active type autofocus (abbreviated as AF hereinafter) apparatus is so generally prevalent that this kind of AF apparatus is mounted on most lens shutter cameras.
The formation of the above mentioned active type AF apparatus based on the generally known triangular distance measuring method shall be explained in the following with reference to FIG. 12. A light emitted from an infrared ray emitting diode (abbreviated as an IRED hereinafter) 3 is condensed and radiated by a light projecting lens 1 toward an object to be photographed and the reflected light is made by a light receiving lens 2 to form an image on a well known position sensitive device (abbreviated as a PSD hereinafter) 4 made of a one-dimensional semiconductor position detecting device. As a result, this PSD 4 will provide an output to be separated into light current signals I.sub.1 and I.sub.2 which will be fed to a later described IC for AF.
Here, by the principle of the triangular distance measuring method, the entrance position x of the reflected light on the PSD will be a function of the distance l to the object as in the following formula: EQU l=s.multidot.f/x (1)
wherein s represents a distance (base line length) between the main points of the light projecting lens 1 and light receiving lens 2, f represents a focal distance of the light receiving lens 2 and the PSD 4 is arranged in this position. This PSD 4 outputs the two light current signals I.sub.1 and I.sub.2 which are functions of the entrance position x of the distance measuring signal light. Now, when a line parallel with a line connecting the light emitting center of the IRED 3 and the main point of the light projecting lens 1 is extended from the main point of the light receiving lens 2, if the point of crossing the PSD 4 is taken at a point of a length a from the end on the IRED side of the PSD 4, the total signal light current is represented by I.sub.P0 and the total length of the PSD 4 is represented by t, EQU I.sub.1 ={(a+x)/t}I.sub.P0 ( 2) EQU I.sub.2 ={t-(a+x)/t}I.sub.P0 ( 3)
will be made and therefore, from the above mentioned formulae (2) and (3), 1/l can be determined by ##EQU1##
That is to say, in the above mentioned formula (4), as the total length t of the PSD 4, the length a from the end on the IRED side of the PSD 4, the base line length s and the focal distance f of the light receiving lens 2 are respectively constants, if the light current ratio I.sub.1 /(I.sub.1 +I.sub.2) is determined, the reciprocal 1/l of the object distance will be able to be determined.
The above is the summary of a known one-point distance measuring apparatus.
However, in most of the conventional AF cameras of this kind, the AF operation is made by one IRED and therefore only the distance to any one point to which the projected light from the same IRED is radiated has been able to be measured. That is to say, the distance has been able to be measured only at one point or mostly in the central part within the picture. Therefore, if the main objects to be photographed deviate from this point as in the case that the main objects 11a and 11b are not present in the position upon which the distance measuring beam P is projected in the picture to be photographed, that is, as shown in FIG. 5A, a so-called "central blank" state will be made and the AF apparatus will focus another object or the background, that is, the infinite distance and will take a photograph out of focus of the main objects.
Therefore, in order to eliminate this problem, a multipoint distance measuring apparatus wherein, as shown in FIG. 5B, distance measuring spot lights are radiated to a plurality of points, for example, three points L, C and R in the picture to be photographed and the object distances are determined from these reflected lights to prevent the "central blank" is suggested by Japanese Patent Applications Laid Open Nos. 9013/1983 and 76715/1983. In this kind of multipoint distance measuring apparatus, the possibility of the object being at the nearest distance point among a plurality of measured distance point informations is so high that the photographing lens is driven and controlled to the nearest distance.
However, in the distance measuring apparatus wherein a distance measuring spot light is radiated to an object to be photographed, if all the spot light having a limited expanse is radiated to the object, the distance will be accurately measured but, if the spot light hits the object at the edge, only a part of the spot light will be reflected and the distance will be mis-measured. This mis-measurement of the distance shall be explained more particularly in the following with reference to FIGS. 9A to 11B.
FIG. 9A shows a spot light 21 on an object 11 to be photographed. FIG. 9B shows a reflected light 21a on a PSD 4. As shown in this FIG. 9A, in case the splot light perfectly impinges upon the object 11, the reflected light 21a will ideally impinge upon the PSD 4 which will output a right light current signal from its gravity position and the gravity position will move from the position corresponding to the infinite distance on the PSD 4 to the position corresponding to the extremely near distance in response to the object distance.
However, as shown in FIGS. 10A and 11A, in case the object 11 is a little deviated in the lateral direction from the position of the spot light 21 shown in FIG. 9A and the spot light is not perfectly reflected, a light current signal will be output from the gravity position of only the reflected light and therefore, even for the object 11 located at the same distance as in FIG. 9A, in the case of FIG. 10A, the gravity position will move to the near distance side on the PSD as shown in FIG. 10B and a light current signal deviated to the near distance side more than in FIGS. 9A and 9B will be output. On the contrary, in the case of FIG. 11A, as shown in FIG. 11B, a light current signal deviated to the far distance side will be output.
In case one spot light is received with one PSD, the amount of this mis-measurement of the distance may be comparatively small but, in the case of a multipoint distance measuring apparatus receiving a plurality of spot lights with one PSD, the moved amount on the PSD from the infinite distance to the extremely near distance of one spot light will be so small that the mis-measurement of the distance by the above mentioned "spot light deviation" will become large. Further, the mis-measurement of the distance by this "spot light deviation" is likely to be produced on each spot light. This likelihood will be higher in the multipoint distance measuring apparatus by the number of the many points than in the one-point distance measuring apparatus, the distance measurement will result in outputting a light current signal deviated to the near distance side and a photograph that is out of focus will be made.